The sensitivity of ageostrophic circulations to model resolution: a case study using NGM forecasts of ERICA IOP-4

The effect of model resolution on the ageostrohic circulations and dynamics of a rapidly deepening extratropical cyclone that occurred during the Intensive Observing Period (IOP) 4 (4-5 January 1989) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is examined using the psi-vector technique proposed by Keyser, et al. (1989). A comparison of forecasts made by the National Meteorological Center's Nested Grid Model (NGM) using operations (C-grid_ and double-resolution (D-grid) versions indicated pressure and frontal evolution differences between the two forecasts. The IOP-4 cyclone was observed to deepen by 60 mb in 24 h to a central pressure of 936 mb. The NGM C-grid forecast attained a central pressure at 0000 UTC 5 Jan of 959 mb, while the D-grid forecast attained a central pressure of 949 mb. Differences in this laterally coupled jet streaks between the C- and D-grid forecasts enabled the D-grid forecast to depict stronger transverse ageostrophic circulations that produced stronger ascent and greater spin-up of the low-level cyclone.